The present invention relates to a degassing apparatus for use with polymerisation reactors, for example for use in making polyethylene.
Polyethylene is typically manufactured according to the Phillips process in a slurry system. In this system high purity ethylene is fed to a loop reactor. A low boiling point hydrocarbon such as isobutane is used to dissolve the ethylene monomer and to suspend catalyst and polymer particles within the reactor. Ethylene gas, the dilluent and powdered catalyst are fed continuously into the loop reactor in which the contents are rapidly circulated by means of a pump. The reactor is typically maintained at a temperature of the order of 100° C. and a pressure of the order of 4 MPa. As the process continues, polymer particles start to form and the larger ones precipitate and enter a settling zone from which concentrated slurry is discharged. The production system is continuous such that the product is removed at the same rate at which it forms.
Because of the high pressure within the reactor it will be appreciated that the discharged slurry contains ethylene which it is desirable to recycle in order to avoid waste. The polymer must also be separated from other materials. The slurry is therefore supplied from the loop reactor to a so-called flash tank or flash drum which is maintained at a much lower pressure than the reactor. This results in the ethylene and low boiling point dilluent flash-vaporising overhead. The monomer gas and the dilluent is then transferred to a compressor and reused.
However, it is necessary to purify this “flash gas”, and in particular to remove small particles known as “fines” therefrom as these could damage the compressor. For this reason the vapour is passed firstly to a cyclone and from there to a bag filter. Both the cyclone and the bag filter are connected to collection tanks known as “drop out pots” which collect solid matter. This matter is then fed via rotating valves (which prevent gas flowing therethrough) to a dryer called a “fluff dryer” along with the slurry from the flash tank. The purified gas is then fed via a so-called guard filter to the flash gas compressor. The guard filter is intended merely to protect the compressor in the event of a failure elsewhere in the system.
The fluff dryer dries the matter fed to it to leave polymer powder. Additives are added if required and the powder is then sent on to an extruder in which it is melted and formed into pellets.
The system described above is satisfactory in most circumstances. Over a period of time, the filters, which are typically bag filters, become blocked and the process must be shut down so that these may be cleaned or replaced. Normally, this is not a serious problem because this maintenance is only required infrequently.
However, it has been found that in certain circumstances the filter and sometimes the cyclone clog up after only a short period of time. It has been found that this occurs when polymer having a high comonomer (such as 1-hexene and 1-octene) content is being produced. Hexene may be added to the olefin feed in order to control density of the polymer and it is believed that condensed hexene and oligomers make the overall product sticky. This in turn causes rapid clogging of the cyclone and bag filter. Comonomer 1-hexane also wets the filters which increases this clogging. It is thought that the higher hexene content results in lower temperatures which in turn create an increased amount of condensation which clogs the filter and cyclone.
As a consequence of this, it is only possible to have comparatively short production runs of high hexene content polymers with a practical limit of around 2000 tons. After this, low hexene content products have to be produced in order to clean the cyclone and bag filter. It will be appreciated that this is a serious problem if large volumes of high hexene content product are to be produced.
According to one aspect of the invention there is provided a degassing apparatus for use in association with a polymerisation reactor, the apparatus comprising a flash tank into which slurry from the reactor may be discharged and within which monomer(s) and/or dilluent in the slurry may be vaporised to form gas; and a filter unit for removing suspended particles from the gas; wherein the filter unit is directly connected to the flash tank without an intervening cyclone or the like.
Thus, in contrast to the conventional arrangement, the cyclone located between the flash tank and the filter has been eliminated. Conventionally it was believed that the presence of a cyclone was essential in order to prevent excessive liquid or solid matter reaching the filter and thereby causing the filter to clog. However, the inventors have found that, surprisingly, improved operation is achieved, at least when producing high hexene content polymers, by feeding the flashed material directly to a filter unit. This maintains a high temperature and thereby prevents, or at least reduces, clogging of the filters. It will also be appreciated that this arrangement results in a significantly reduced component count, shorter piping runs and a generally simpler arrangement.
As discussed, this arrangement is advantageous in itself since by reducing clogging the need to shut down the system in order to clean or replace filters is reduced. However, the above described arrangement lends itself to a particular further advance over the prior art system whereby there may be provided a plurality of filter units each directly connected to the flash tank. Valve apparatus may then be provided such that one of the filter units may be isolated for cleaning, removal or replacement whilst the other filter(s) remain in operation. In the present context “valve apparatus” means any appropriate flow controlling apparatus. Thus, the apparatus may be designed to operate using one or the other of the filter units at any one time, or it may be arranged to use both filters during normal operation with the option of isolating one or more of the filters for maintenance.
It will be appreciated that the filter units are preferably located as close as possible to the flash tank in order to keep the pipe runs to a minimum in order to reduce temperature drop. Each filter unit may be provided with a conduit to direct solid matter therefrom to a dryer which may be the same dryer to which material is directed from the flash tank. However, in one embodiment the filter unit(s) is/are mounted at an upper part of the flash tank and are arranged such that particles thereby removed from the gas may fall into the flash tank itself. They then pass into the dryer along with solid matter from the tank.
This arrangement is in itself regarded as providing an invention and therefore from another aspect the invention provides a degassing apparatus for use in association with the polymerisation reactor, the apparatus comprising: a flash tank into which slurry from the reactor may be discharged and within which monomer(s) and/or dilluent in the slurry may be vaporised to form gas; and a filter unit for removing suspended particles from the gas; wherein the filter is mounted at an upper part of the flash tank and is arranged such that particles thereby removed from the gas may fall into the flash tank.
It will be appreciated that this arrangement is particularly advantageous in that it maintains the temperature in the region of the filters, thereby reducing the clogging problem, and greatly simplifies the structure and reduces the component count.
The filters may be mounted on top of the flash tank with suitable conduits which must be of a suitable diameter to enable matter to fall back into the flash tank in use against a flow of gas into the filters. Alternatively, the filter unit(s) may be formed integrally with the flash tank itself as a single structure.
As previously discussed, a plurality of filter units may be provided and they may be arranged such that one or more may be isolated for maintenance, cleaning of filter materials, replacement etc. This applies equally whether the filters are integral with the flash tank or separate therefrom.
This arrangement is also regarded as being inventive in its own right and therefore viewed from a further aspect the present invention provides a degassing apparatus for use in association with a polymerisation reactor, the apparatus comprising a flash tank into which slurry from the reactor may be discharged and within which monomer(s) and/or dilluent in the slurry may vaporise to form gas; and a plurality of filter units for removing suspended particles from the gas; whereby the filter units are arranged such that one of the filter units may be isolated for cleaning, removal or replacement whilst the other filter(s) remain in operation.
Although it is possible to use any conventional type of filter in this context, it is preferred that the filter units comprise bag filters.
Additionally or alternatively, the filter units may comprise sintered metal filters such as those available from Pall Corporation. These have the advantage that they can be cleaned, either in situ or after removal, by ultrasound, back flushing when above a certain pressure etc. If it is desired to clean the filters in situ by back flushing then appropriate pipework and valving would be provided to generate the necessary back flow.
Alternatively, if the filters are to be removed for cleaning then the sintered metal filters are particularly advantageous in that the material can be burnt off without damaging the filters.
Downstream of the filter unit(s) the apparatus of the invention may be entirely conventional and therefore there may be further provided a compressor for compressing the gas and a guard filter to protect the compressor from any particles not removed by the filter unit(s). Furthermore, there may be provided a dryer such as a fluff dryer and/or a purge dryer for drying solid matter separated from the vaporised gas in the flash tank and/or for receiving solid matter from the filter unit(s).
It will be appreciated that the invention also extends to a method of degassing a slurry from a polymerisation reactor comprising supplying the slurry to the apparatus as discussed previously and allowing the apparatus to degas the slurry in the manner described and subsequently separating the monomer gas and solid matter.
Furthermore, the invention extends to a method of producing a polymer whereby monomer, catalyst and other reactant(s) are supplied to a polymerisation reactor and the resultant slurry is supplied to a degassing apparatus as previously described whereby the polymer is separated from monomer gas. The polymer may then be sent as a powder to an extruder in which it is melted and formed into pellets.